This invention relates to golf clubs, and, more particularly, to golf clubs which are designed to provide more accurate off-center hits.
It is commonly assumed that when a golf club strikes a golf ball off center, i.e., at a point spaced from the center of gravity of the club toward the toe or heel, the club rotates about a vertical axis which extends through the center of gravity. When the club strikes the ball at a point above or below the center of gravity, i.e., toward the top or sole of the club, it is assumed that the club tends to rotate about a horizontal axis which extends through the center of gravity.
The foregoing assumptions have resulted in clubs being designed with increased polar moments of inertia about the vertical and horizontal axes in order to reduce the rotation of the club. The moment of inertia about the vertical axis may be increased by concentrating weight of the club in the toe and the heel. The moment of inertia about the horizontal axis may be increased by concentrating weight of the club in the sole and the top.
Wooden clubs, particularly the driver, are conventionally provided with bulge and roll in order to compensate for excessive gear effect. Bulge is convex curvature in the face of the club which can be seen when the face is viewed from above or below. The curvature extends about a vertical axis. Roll is convex curvature in the face which can be seen when the face is viewed from the side. The curvature extends about a horizontal axis. Gear effect in wooden clubs is well known. When a wooden club, e.g., a driver, strikes a golf ball at a point which is offset from the center of the face. i.e., toward the toe or heel of the club, a spin is imparted to the ball. A toe hit on a right-handed club will provide a counterclockwise or hook spin, and a heel hit on a right-handed club will provide a clockwise or slice spin.
The gear effect spin is created by a wooden club because the center of gravity of the club is spaced a substantial distance behind the striking face of the club. When the club strikes a ball on an off-center hit, the clubhead rotates about its center of gravity. The rotation of the clubhead in one direction, e.g., clockwise for a toe hit, causes the ball to rotate in the opposite direction, i.e., counterclockwise for a toe hit. An opposite rotation is imparted to the ball because the clubhead and ball rotate together much like two enmeshed gears. On a heel hit, the club rotates counterclockwise, and a clockwise or slice spin is imparted to the ball.
The clockwise rotation of a wooden clubhead on a toe hit opens the face of the club and causes the ball to fly initially to the right of the intended line of flight. However, the counterclockwise or hook spin imparted to the ball by the gear effect will cause the ball to curve back toward the intended line of flight. In most wooden clubs the gear effect spin more than compensates for the effect of the open face, and the ball would hook to the left of the intended line of flight. For this reason the striking face of a wooden club is provided with bulge, i.e., a curved or convex face. The bulge tends to make a toe hit fly to the right and a heel hit fly to the left. The bulge spin and the gear effect spin are advantageously adjusted so that a ball hit on either the toe or the heel lands approximately along the intended line of flight.
Similarly, when a wooden club strikes a ball above or below the center of gravity, the rotation of the clubhead about the center of gravity imparts gear effect spin on the ball which causes the ball to curve downwardly for a high hit and to curve upwardly for a low hit. The roll curvature is intended to compensate for excessive gear effect spin on high and low hits.
U.S. Pat. No. 3,625,518 describes orienting the bulge curvature of a wooden club about an axis which extends parallel to the axis of the shaft and orienting the roll curvature about an axis which is perpendicular to the first axis. The patent does not precisely describe the manner in which the clubhead rotates at impact. Instead, the orientation of the bulge and roll axes is said to compensate for deviation of the impact from the sweet spot because any tendency to swing out is accompanying by a tendency to raise the club and vice versa. The patent also states that the swing of the club at impact is in a plane passing through the target line and parallel to the shaft.